Open Access

Over the past three decades, luminescent nanocrystals have evolved from a scientific curiosity into a versatile class of functional materials with applications spanning photonics, bioimaging, sensing, energy conversion, and quantum technologies. Throughout this development, Markus Haase has played a defining role in shaping the field through his pioneering contributions to colloidal semiconductor quantum dots and, in particular, lanthanide-doped upconverting nanocrystals. His work established fundamental synthetic strategies, provided mechanistic insight into particle formation and optical processes, and set new standards for reproducibility, characterization, and scientific rigor. This commemorative lecture brings together the perspectives of colleagues, former doctoral researchers, collaborators, and long-standing friends from the scientific community to celebrate Markus Haase's remarkable career and lasting scientific legacy. Beyond highlighting key milestones and breakthroughs, the presentation reflects on the qualities that have distinguished his work: intellectual curiosity, creativity, uncompromising scientific integrity, and the willingness to pursue challenging and unconventional research directions. Equally important, it acknowledges the personal values that have inspired generations of researchers—his honesty, loyalty, commitment, passion for science, and distinctive sense of humor. Together, these contributions have not only advanced the field of luminescent nanocrystals but have also left a lasting impact on the people and collaborations that continue to shape it.

The use of engineered nanoparticles of different size, shape, and composition is continuously increasing in life and materials sciences. This calls for methods and reference materials enabling the reliable and accurate determination of nanoparticle size, particle size distribution, shape, number concentration, degree of aggregation and agglomeration in different environments as well as for nanoparticle dispersibility and stability. We are currently building up and exploring a platform of lanthanide-based nanocrystals (LnNCs) and other functional nanoparticles with application-specifically tuned size, shape, composition, architecture, optical properties, and surface chemistry for emerging applications in life sciences and as reference and test materials. As a prerequisite for the broad applicability of these nanomaterials, we assess simple, robust, and easily up-scaleable synthesis protocols for LnNCs with defined morphologies and tunable optical properties [1], and the short-term and long-term stability of LnNCs with selected surface coatings in aqueous environments under different application-relevant conditions.[2] In addition to sensing and encoding applications, we are currently exploring the applicability of our LnNCs which can be prepared in large amounts in different shapes,. i.e., as spheres, bypyramids, or rods with excellent control of size and size distribution, as reference material candidates for sizing methods such as electron microscopy and small-angle X-ray scattering (SAXS) as well as for determination of particle number concentration. For spherical NPs, particle size can be easily determined from measurements of the diameter of by electron microscopy providing 2D information on a large number of dried nanoparticles deposited in vacuo on a solid substrate. However, for NPs with a different shape, this 2D-projection can be misleading. Here, new correlative approaches are needed for the complete 3D characterization for particles with a more complex shape [3] and non-spherical reference particles for establishing such methods and the implementation and validation of the respective data fitting routine. In this context, we are currently exploring the potential of differently sized functional nanoparticles as test and reference materials for size and particle number concentration (PNC) by SAXS and interlaboratory comparisons involving different SAXS expert groups.[4]

Many commonly used dye classes suffer from strong overlap of their absorption and emission spectra, favoring reabsorption and hampering the combination of several dyes for multi-analyte sensing with single wavelength excitation. This can be overcome by increasing the energy difference between absorption and emission using donor–acceptor dyes with charge-transfer processes. A neglected concept to finetune the Stokes shift is meta-substitution, which we exploited to design a single-benzene fluorophore exhibiting the largest Stokes shift of a zwitterionic compound. The meta-substitution of permanently-charged donor and acceptor groups provides a Stokes shift of >10 000 cm−1 (1.24 eV), absorbing light in the UV-region at 375 nm and emitting yellow-orange light at 605 nm. Relative to para substitution, the orbitals are primed for more effective intramolecular charge-transfer and more Energy is dissipated by structural reorganisation upon excitation, stemming from greater excited-state antiaromaticity. The large Stokes shift is retained by p-extended derivatives, thus meta-substitution of zwitterionic groups is a general way to design organic fluorophores with small spectral overlap.

Engineered nanomaterials (NM) of different sizes, shape, chemical composition, and surface chemistry are increasingly used for many key technologies of the 21st century and consumer products. Decisive for most NM applications are their specific surface properties, which are largely determined by the chemical nature and amounts of ligands and functional groups (FGs) on the NM surface and strongly affect NM properties such as charge, hydrophilicity/hydrophobicity, reactivity, function, stability, and processability and thus their potential impact on environment and biological species. In this context, different methods for analyzing surface FGs on NMs are presented including their measurement principles, advantages and limitations and examples for their usage.

Nano-enabled advanced materials (NAMs) and nanoparticles (NPs) with various chemical compositions and surface functionalities are routinely fabricated for industrial applications such as medical diagnostics, drug delivery, sensing, catalysis, energy conversion and storage, opto-electronics, and information storage. Their function, interaction with biological species, and environmental fate are largely determined by surface functionalities. Reliable, reproducible, and standardized surface characterization methods are thus vital for NP quality control, and mandatory to meet increasing safety concerns. Also, industry, international standardization organizations such as ISO and IEC, regulatory agencies, and policymakers need validated and standardized measurement methods. However, methodologies for determining NP surface properties, including the amount, chemical composition, and homogeneity of surface functionalities and coatings are largely non-standardized. Suitable methods for determining NP surface functionalities include advanced techniques such as traceable quantitative nuclear magnetic resonance (qNMR), X-ray electron spectroscopy (XPS), and time of flight secondary ion mass spectrometry (ToF-SIMS) and simpler optical and electrochemical methods.[1,2] The latter less costly methods are often used by industry, e.g., for quality control. To validate these methods and determine uncertainties, international interlaboratory comparisons (ILC) are needed, thereby also establishing test reference materials and reference nanomaterials, providing benchmark values, and reference data for surface chemistry measurements.[3] These needs are addressed by the European metrology project SMURFnano involving 12 partners from different National Metrology Institutes, designated institutes, research institutes, two university groups as well as one large company and one SME producing NPs. Here we present first results of multi-method characterization studies and performed with surface-functionalized silica nanomaterials from different sources, covering monodisperse silica NPs as well as more challenging high-volume industrial materials such as fumed silica. Special emphasis is dedicated to the development of harmonized protocols for ILCs on qNMR and XPS under the roof of VAMAS and CCQM that are currently running.